1. Field of the Invention
The present invention relates to the fabricating of staircases and more particularly relates to a method and apparatus for the preparation of staircase stringer portions with selectively spaced tread assembly positions that will define horizontal stair-step tread positions during use.
2. General Background
Staircases normally include one or more longitudinally extending members known as "stringers". The stringers support the horizontal part of the step in the staircase, known as the "tread". A stringer can optionally support a vertical part of a step, the riser.
During the construction of staircases in a modular fashion, a problem arises in that each staircase to be manufactured may not be of the same dimension because of preexisting construction, and/or the custom needs of a particular home or building. In such instances, the length and/or inclination of the stringer can change. Further the horizontal and vertical distances between the treads of the stair-step positions can vary. These changes in configuration can be minute from one manufactured staircase to the next, but tread position changes of even a few inches eliminates the ability to repeatedly manufacture the same staircase in high volume without expensive custom carpentry or custom welding.
Various machines have been patented which relate to machines that work metal, wood and work pieces with multiple material working stations. The Pearson U.S. Pat. No. 2,939,499 entitled "Automatic Machine For Routing Right and Left Hand Stair Stringers" provides an apparatus that uses a carriage upon which are mounted motors that are equipped with routers. The carriages are slideably moveable upon guide rails and powered by a fluid operative cylinder and piston. A mechanism is provided for adjusting the guide plates about their pivots for changing the riser-rung ratio for the stair stringers routed by the machine. The adjustment mechanism includes a block which slides along a fixed guide extending from the back to the front of the main frame of the machine. A lead screw threads through the block and this lead screw is held against axial movement by a thrust bearing. The screw is rotated by applying a crank to the square end at the front of the machine. This apparatus requires an advancement of the stringer to each stair step position so that the router can perform each of the multiple cuts as the stringer is advanced alongs its length. this is unlike the present invention wherein all the cuts are made while the work piece is held by the machine. Intervention by an operator to advance the stringer into multiple successive positions is not required. Additionally, the present invention can quickly adjust the spacing of the tread position on the stringer for each work piece. thus, custom staircases of variable dimensions and with variable tread spacing can be manufactured on a high volume basis.
Another patent showing material working machines using a plurality of work stations such as routers is the Onsrud U.S. Pat. No. 2,168,234 entitled "Grooving Machine".
3. General Discussion of the Present Invention
The present invention provides a staircase stringer fabricating apparatus that can form the stair step tread positions on the stringer as a single operation after the stringer work piece is mounted upon the machine. No operator intervention is required in order to advance the stringer from one stair step position to the next. With the present invention, the cuts (or welds) are preferably made simultaneously by a plurality of work stations as the work piece moves relative to the work stations and upon the frame.
In the preferred embodiment the work stations carry wood routers and the work piece is an elongated wooden stringer. However, it should be understood that variations of the preferred embodiment would include for example the use of a metal stringer work piece and welding torches at each station for welding treads to the stringer.
The preferred embodiment of the present invention includes a structural machine frame upon which is moveably mounted a carriage which supports the work piece. The frame also supports multiple material working stations which prepare the work piece with selectively pre-spaced tread assembly positions that define the horizontal stair-step positions for the staircase treads during use. The stair step positions are preferably formed responsive to a simultaneous movement of the work piece and router work stations with respect to each other. A track is provided on the frame for guiding the movement of the work piece carriage and the working stations with respect to each other along a controlled path which gives precision to the stair step positions formed on the work piece. Thus, the work piece is formed with stair step positions which can be, for example, parallel grooves, each having corresponding, pre-selected, equally spaced positions with respect to the work piece. The cut grooves are preferably of a constant length, and have a constant angular orientation with respect to the longitudinal axis of the work piece.
In the preferred embodiment, the work stations are moveably mounted upon the frame so that spacing between adjacent work stations can be varied as desired. In the preferred embodiment, the spacing between adjacent stations can be varied simultaneously in controlled, proportional spacing so that the spacing is always the same between adjacent work stations. The spacing is selectively variable with respect to the length of a particular work piece being prepared. Thus, for example, a stringer of any length can be prepared with stair step tread positions which are equally spaced at any value between about six (6") inches, and eight (8") inches apart which might be necessary depending on angular orientation of the stringer and the application or use of the stair.
In the preferred embodiment, the router forms a dovetail groove in the stringer to which a stair step tread can be attached.
The work stations are preferably router carriages mounted upon the frame on one or more elongated, smooth, common rails. Each router carriage is moved upon the rails by an elongated cable section. Each cable is endless, and is wound upon four sets of sheaves including a large multiple diameter sheave assembly. In the preferred embodiment, the spacing between adjacent work stations is maintained equal, yet may be simultaneously adjusted with the multiple diameter sheave assembly. The sheaves of multiple diameter are mounted for rotation on a common axis drive shaft, and an endless cable is wrapped about each sheave. Each cable extends the length of the machine frame so that when the multiple diameter sheave assembly is rotated by the common shaft, two portions (upper and lower) of the same endless cable move in opposite directions with respect to each other. These upper and lower portions of each endless cable are attached respectively to a pair of corresponding work stations. These corresponding work stations move apart, or together the same distance when the multiple diameter sheave assembly is rotated. A single central work station is unattached to a cable and remains static. the first pair of work stations on each side of the static station are mounted to the cable wrapped upon the smallest sheave and thus move the smallest distance when the shaft carrying all the multiple diameter sheaves is rotated. Conversely, the work stations mounted upon the endless cable which is wound about the largest sheave move the greatest distance during rotation.
In the preferred embodiment, the central work station is static and the work stations on either side of the central work station are connected to the cable wound upon the smallest pulley. Each successive pair of work stations moving in a direction away from the static work station are connected to an endless cable wound upon the next largest diameter sheave. Work stations that are farthest from the central static work station are attached to endless cables which are wound about the larger sheaves. Rotation of the cental drive shaft and all of the sheaves simultaneously produces an adjustable spacing of all the work stations with respect to the frame such that, even though the distance between any two adjacent stations is varied, the distance between all adjacent stations is always the same. For example, if the stations are all spaced seven (7") inches apart and the variable diameter sheave assembly rotates a small amount. Spacing between all of the stations might be decreased to six and three quarter (6-3/4") inches. To accomplish this, the diameter of the sheaves are selected so that the circumference of each is an integer multiple, starting at "2", of the circumference (or diameter) of "X" inches, the remaining sheaves would have a circumference (or diameter) of "2X", "3X", "4X", etc. respectively.